Centrifugal force separators have been used successfully for decades in the area of mills and fodder mills. The greatest advantage of traditional cyclone separators lies in their simple method of construction and their relatively low air resistance. In the general case, the cyclones are used with a vertical axis and in rare cases are slightly inclined. The separated substances are collected in the lower area of the centrifugal force separator and discharged via a product valve. In the upper peripheral area, the air enters tangentially into the cyclone and, after several vortex motions, leaves it centrally in the uppermost area through the so-called "immersion tube" which projects slightly into the inside of the cyclone.
The main disadvantage of the cyclone lies in its relatively poor efficiency for dust separation. In the cyclone, a plurality of superimposing secondary vortex motions develop which, together with a fluctuating air pressure and varying dust charge, prevents for practical application, a substantial improvement in the degree of separation. A further disadvantage is that, particularly in the area of a mill or a fodder mill when cyclones are used as separators, the exhaust air still has residual dust contents which are substantially above the statutory permissible values. For industrial plants, therefore, the exhaust air of cyclones has to be additionally cleaned via filters before it may be discharged into the open air.
To this day, many proposals for the improvement of cyclone separators have been made, but with a few exceptions these have been unable to achieve success in practice. One of these exceptions is described in the DE-B No. 1,078,859. Here, a centrifugal force separator is used which has a horizontal axis in the form of a twin centrifugal force separator or a primary and secondary separator. The primary separator is constructed in a spiral and approximately circular shape, with the raw gas entering tangentially. The outermost air layer is "peeled off" as it were at the opposite end of the spiral chamber and fed into a substantially smaller secondary separator, in which (similar to traditional cyclone separators) the clean air and the dust are separated at both end sides. An advantage of this separator system lies in the very low pressure loss, but its disadvantage lies in an inadequate degree of separation.
Recently, there has been a noticeable tendency to use the individual cleaning machines which require very large air quantities in return-air operation, for example, in a mill (cf., eg., GB-A No. 1,536,905). However, return-air machines require relatively clean air, for two reasons: if too large a proportion of dust is contained in the return air, there is the danger of a permanent bacterial contamination of the material, especially if this is raw material for human food. If there is a lot of dirt and dust in the return air, the entire machine will become blocked by dust in a short time. Either breakdowns occur frequently or much more cleaning work has to be performed.
Although the quality requirements for the return air do not need to be as high as the statutory regulations for the quality of industrial exhaust air into the open air, the quality requirements for the return air, from experience, are always much greater than could be guaranteed by the efficiency of known centrifugal force separators or cyclone separators.
Starting from this basis, it is the object of the invention to develop a centrifugal force separator for broken grains, husks and dust and other impurities from cereal crops, which has a substantially increased degree of dust separation at only a slight pressure loss, is inexpensively constructed and is suitable for use in return-air systems in particular in combination with other cereal crop cleaning and processing machines.
In a centrifugal force separator of the type mentioned at the beginning, this object is achieved according to the invention in that a pre-separating chamber for an air circulation is provided radially outside the deflecting screen, and an air discharge is provided radially inside the deflecting screen, which air discharge is in flow connection with the pre-separating chamber via air passage channels in the deflecting chamber. As a result of the invention, two chambers are created which can be controlled and are precisely determined as regards flow, by which means a very extensive separation of impurities from the air can be achieved.
In an advantageous further development of the invention, the pre-separating chamber is made circular in cross-section and is arranged directly above a funnelshaped collector which, at its upper side, is divided from the pre-separating chamber by a curved deflecting wall in such a way that air circulation openings remain on both outer sides of the pre-separating chamber.
The centrifugal force separator according to the invention was first of all tested in conjunction with an aspiration channel having a preselected dust charge, with surprisingly good results being obtained.
The tangential raw gas inlet preferably has an inflow arranged in a curved shape and pointing in the same direction towards the pre-separating chamber. In this especially preferred design of the centrifugal force separator according to the invention, the action of the centrifugal force is already well prepared in the inflow of the pre-separating chamber. At the same time, disturbing, "superimposed" turbulence is avoided on entering into the pre-separating chamber, in particular if the tangential raw gas inlet essentially extends over the entire length of the pre-separating chamber.
A raw gas inlet in the upper area of the preseparating chamber, in which the air flow runs in the clockwise direction in the pre-separating chamber and the air flows out of the raw gas inlet from bottom left upwards into the pre-separating chamber, has thus far proved to be the best solution. In this connection, a particularly undisturbed flow and a remarkably effective separation of the impurities from the air are obtained if the deflecting screen has an upper section impermeable to air. At the same time, the air becomes enriched suprisingly quickly with the foreign bodies present in it during the course of a semi-circular motion in the pre-separating chamber in the zone next to the walls, so that this enriched outer partial flow can discharge all foreign bodies when flowing over into the funnel-shaped collector. Two main active forces occur in the collector itself: on the one hand, the foreign bodies fall downwards as a result of gravity; on the other hand, however, the centrifugal force also acts here again, because all of the air flowing into the collector can flow back again into the pre-separating channel on the collector side opposite the inflow point. At the same time, however, individual dust or husk particles being pulled along again cannot be avoided. However, the probability of these particles still being deposited at the bottom in the collector during a subsequent through-flow is very great - as tests show.
The inner partial flow largely freed of foreign bodies enters at the inner side of the curved deflecting wall into the space between the latter and the deflecting screen. At the same time, however, a small quantity of dust particles and injected grains unfortunately cannot be prevented from being pulled along with the inner partial flow. To clean the inner partial flow of these foreign bodies too, various further advantageous embodiments of the invention have proved successful:
Thus, in a quite especially preferred further development of the invention, the deflecting screen only has air passage channels in its lower section. The air passage channels are also advantageously arranged in the deflecting screen in the area of the latter which is facing towards or is opposite the deflecting wall curved in a circular shape. Moreover, the deflecting screen, with particular advantage, has essentially radially arranged guide vanes, that is, arranged transversely to the rotational flow of the air, with, again advantageously, air passage channels between the guide vanes forming a deflecting angle for the air flow of more than 90.degree.. Furthermore, the air passage channels are preferably made such that the aspirated air quantity enters irrotationally into the clean gas outlet.
In a centrifugal force separator according to the invention, it is also of particular advantage if, in front of the funnel-shaped collector, in the area of the raw gas inlet, a channel is provided for returning the air into the pre-separating chamber. The deflecting wall curved in a circular shape is made particularly advantageously if it has a lower boundary with such a spiral shaped portion, the space between the deflecting screen and deflecting wall opens, again advantageously, into the air return channel.
To guide the air such that it is free of cross turbulence, the deflecting screen, preferably in its upper area, is closed over an angle of more than 180.degree.. It is also proposed for the advantageous embodiment of the invention to have the deflecting wall which is curved in a circular shape to start in the area of the horizontal center plane of the deflecting screen and to make it over an angle between 90.degree. and 180.degree..
In a further preferred embodiment of the invention, the raw gas inlet is made as the upper area of a vertical aspiration channel, with the clean gas outlet preferably being connected to a lower inlet, arranged at the aspiration channel, in such a way that the aspiration channel works in return-air operation. The best results for a purposeful selection of a desired grain material category in the aspiration channel and for a subsequent separation of the remaining grain material categories or husks or dust particles in the centrifugal force separator can be achieved if, in a centrifugal force separator according to the invention, a rear wall of the aspiration channel can be adjusted in both its inclination and horizontal direction (thus in a double respect).
The centrifugal force separator according to the invention has proved surprisingly successful when used in combination with an aspiration channel for cereal crops. In this application, all good and heavy cereal grains are to be freed by the aspiration channel of any foreign content (ie. husk parts, dirt, dust, and also broken grains and shriveled kernels and the like). To re-separate the relatively large quantity of foreign content completely and economically from the air has proved to be a great problem in the past, which hitherto could not be solved satisfactorily. Here, the use of a centrifugal separator according to the invention showed for the first time a completely satisfactory separation effect, such as could not even be achieved approximately hitherto.